This invention relates generally to nuclear reactors and more particularly, to coupling apparatus for control rods in nuclear reactors.
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud typically surrounds the core plate and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. The top guide comprises several openings, and fuel bundles are installed through the openings. Control rods are installed from above and operated from below.
A plurality of openings is formed in the bottom head dome so that components, such as control rod drive apparatus, can extend within the RPV. As an example, for a control rod drive apparatus, a control rod drive housing is inserted through the bottom head dome opening and a control rod drive mechanism (CRDM) is inserted through the control rod drive housing. The CRDM is coupled to the control rod. The CRDM facilitates positioning the control rod within the core.
A nuclear reactor core includes individual fuel assemblies that have different characteristics that affect the strategy for operation of the core. For example, a nuclear reactor core has many, up to several hundred, individual fuel bundles that have different characteristics. Such bundles preferably are arranged within the reactor core so that the interaction between the fuel bundles satisfies all regulatory and reactor design constraints, including governmental and customer specified constraints. The core loading arrangement determines the cycle energy, or the amount of energy that the reactor core generates before the core needs to be refueled with new fuel elements. In addition to satisfying the design constraints, the core loading arrangement preferably optimizes the core cycle energy.
In order to furnish the required energy output, the reactor core is periodically refueled with fresh fuel assemblies. The most depleted fuel bundles, which include the bundles with the least remaining energy content, are removed from the reactor. Control rods, containing neutron absorbing material, may also be replaced during refuelings. Typically the control rod is disconnected from the CRDM and removed from the RPV, leaving the CRDM in place.
Control rods control the excess reactivity in the reactor. Specifically, the reactor core contains control rods which assure safe shutdown and provide the primary mechanism for controlling the maximum power peaking factor. The total number of control rods available varies with core size and geometry, and is typically between 50 and 200. The position of the control rods, for example, fully inserted, fully withdrawn, or somewhere between, is based on the need to control the excess reactivity and to meet other operational constraints, such as the maximum core power peaking factor.
The control rod is moved vertically by the CRDM to control excess reactivity. In one known reactor design, horizontal and rotational motion of the control rod is constrained by a control rod guide tube. In such design control rods cannot be rotated even after fuel bundle removal due to the control rod guide tube and supporting lattice structure.
The control rod is connected to the CRDM with a coupling assembly to allow removal of the control rod from the reactor core. In one known reactor design, a bayonet coupling is used, requiring rotation of the control rod to effect uncoupling.
It would be desirable to provide a coupling assembly that precludes inadvertent uncoupling of the control rod from the CRDM, but enables uncoupling of the control rod from the CRDM without rotation of the control rod. It also would be desirable to enable uncoupling of the control rod from the CRDM from above the reactor core without removal, rotation or maintenance of the CRDM from below the reactor.
In an exemplary embodiment, a control rod apparatus includes a control rod, a CRDM, and a coupling assembly. The control rod includes at least one blade and a longitudinal tube. The CRDM includes an index tube with a bayonet head secured to one end of the index tube. The coupling assembly includes a bayonet socket sized to receive the bayonet head. The coupling assembly also includes a shaft extending from the bayonet socket through the longitudinal tube of the control rod. A handle extends from an end of the shaft opposite the bayonet socket. Rotation of the handle rotates the shaft and the bayonet socket, with substantially no rotation of the control rod.
In use, the control rod, with the coupling assembly in the longitudinal tube, is lowered onto the CRDM so that the bayonet head is received into the bayonet socket. The handle is rotated, rotating the shaft, and thus the bayonet socket. The control rod does not rotate. In an exemplary embodiment, about 45 degrees of handle rotation fully engages the bayonet socket to the bayonet head, and aligns the handle with a control rod blade.
The above-described coupling assembly facilitates control rod removal, inspection and replacement for reducing out of service maintenance periods. In addition, the above-described coupling assembly facilitates improved reliability of the control rod apparatus.